Efficiency analysis for a manganese-oxide-based thermochemical cycle

A thermochemical cycle for the solar production of hydrogen is proposed. The cycle includes: (1) the conversion of solar to chemical energy by the thermal reduction of manganese(III) oxide to manganese(II) oxide at temperatures below 1900 K, (2) the production of hydrogen by reacting manganese(II) oxide with sodium hydroxide, and (3) the separation of manganese oxide from sodium hydroxide by a hydrolysis reaction. The maximum exergy efficiency of the cycle was calculated to be 74%. Efficiencies of up to 50% were calculated when only partial heat recovery in the solar step was considered. Eventually, the introduction of conventional techniques for particular unit operations yielded efficiencies between 16 and 22%. (C) 1999 Elsevier Science Ltd. All rights reserved.